Also, the save file actually contains the map data as well, so sending someone a save file will work fine even if they don't have the map.

As for performance "killers", it depends a lot on what's going on in your game. A large amount of dynamic objects (stockpiles, bear traps, torches/lanterns), or lots of terrain manipulation (quarries, landscaping, etc).

All of these have been worked on for the 0.8 update, performance should be better.